Epoxy resins have excellent heat resistance, electrical insulation, chemical resistance and mechanical properties, and are widely used in the fields of, for example, paints, adhesives, sealing materials, and structural materials.
Recently, epoxy-carbon fiber composite materials (CFRP) have come into use as structural materials for aircraft, spacecraft, trains, automobiles, and other vehicles, and as sporting goods such as golf club shafts, fishing rods, and skis, because of its mechanical strength and modulus of elasticity comparable to or superior to metals and its light weight.
The polyepoxy compounds used as the matrix resin for CFRP include diglycidyl ether of bisphenol A (e.g., Epikote 828 and Epikote 1004, products of Yuka Shell Epoxy Co., Ltd.), polyepoxide of aminophenol (e.g., ELM-120, a product of Sumitomo Chemical Co., Ltd.), tetraepoxide of methylenedianiline (e.g., YH-434, a product of Toto Kasei Co., Ltd.), cresol novolak polyepoxide (e.g., Epikote 154, a product of Yuka Shell Epoxy Co., Ltd.), and o-cresol novolak epoxide (e.g., EOCN 104S, a product of Nippon Kayaku Co., Ltd.).
The cured products obtained from these polyepoxy compounds have sufficient heat resistance, but they still need improvements in flexibility and impact resistance when used as CFRP.
It is known that a polyepoxy compound having a spiroacetal ring provides a cured product having good flexibility. For example, U.S. Pat. No. 3,128,255 discloses a polyepoxy compound of the formula ##STR1## The cured product obtained from this compound has a heat distortion temperature of 147.degree. to 170.degree. C., and lacks heat resistance when used as CFRP resin.
U.S. Pat. Nos. 3,347,871 and 3,388,098 also disclose a polyepoxy compound of the formula ##STR2## wherein Y is H, Cl or CH.sub.3, and n is an integer of 0 to 2, which is obtained by (A) reacting pentaerythritol with a monohydric phenol having an aldehyde group at the para position with respect to the phenolic hydroxyl group, and (B) reacting the resulting dihydric phenol with epichlorohydrin.
This polyepoxy compound provides a cured product having excellent heat resistance and impact resistance but needs improvements in flexibility. In addition, the compound has poor solubility in conventional solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, toluene, ethyl acetate, and tetrahydrofuran. Thus, it is not suitable for prepreg and paint applications in which solvent dilution is necessary.